In modern gas turbines, especially high performance turbine engines as in aircraft, the turbine must operate at a very high temperature. For the blades, both stationary and moving, to have an adequate life under such extreme temperature conditions, for example, a turbine inlet gas temperature of 2400.degree.F, the blades must be very thoroughly and effectively internally cooled. The best of the known superalloys used for these blades are limited to operation at a temperature several hundred degrees below their melting points but the maximum operating temperature is set as high as possible.
To make the blades hollow with adequately shaped coolant passages therein, the blades are cast in a mold having a core therein to form the cavity in the cast blade. Such core is fragile, subject to distortion and partial disintegration during the casting operation, and must be completely removed when the cast blade has cooled. Complete removal necessitates a core material that disintegrates readily in a solution that will not react with the blade alloy. Even then, inspection of the shape of the cavity and the passages formed therein is almost impossible. The entire problem is aggravated when the blades are made with a columnar crystalline structure in the alloy, as the casting process takes a longer time with resultant greater possibility for mold or core deterioration or distortion. Such columnar structure is described and claimed in VerSnyder U.S. Pat. No. 3,260,505 or, a particular form of columnar structure, single crystal, in Piearcey U.S. Pat. No. 3,494,709.
Blades are cast by the lost wax process in which a wax pattern is coated with mold material to the desired thickness and when the mold is then cured the wax melts out leaving a cavity in the mold the shape of the finished blade. The mold with a growth zone at one end and a filling spout and passage at the other frequently also a part of the lost wax process, is then filled with alloy in producing the cast blade. If the blade is hollow, the wax pattern has inserted therein a core that is the shape of the blade cavity and this core remains in the mold when the wax melts out.
Adequate inspection of the inner surfaces of the mold, and of the location and completeness of the core prior to the casting process is impossible because of the one-piece mold. Thus the casting must be made without knowing that the blade surfaces, even the external surfaces will be satisfactory.
When blades are cast in this manner, the core material must be leachable so as to remove the core from the finished casting. Thus, the materials available for the core are extremely limited and most of the usable materials are not high strength materials nor can they readily withstand the heat from the alloy being cast. Thus, many castings are unacceptable because of core failure. Again, inspection of the cored surfaces of the finished blade is very difficult if not impossible.